Lens blocking and constant center thickness system

ABSTRACT

A base lens blocking system comprises a blocking member formed of a flange with a rearward directed shank received in a collet of a spindle, preferably, a dead length, recessed collet. The front surface of the flange contains a lens blank engaging region which can be recessed. The lens blank is applied to the flat, front surface of the flange by means of a disc of adhesive tape, either a thermoplastic adhesive or a double-sided pressure sensitive adhesive tape or a very thin film of wax applied to a heated surface of the flange under a pressure from 5-20 pounds. A batch of blocked lens blanks can be cut with base curves and processed to finished lenses without measuring the center thickness of the base curves of each lens.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of our U.S. application Ser.No. 07/676,762, filed Mar. 27, 1991, now U.S. Pat. No. 5,205,076, issuedApr. 27, 1993, and U.S. application Ser. No. 07/966,140, filed Oct. 26,1992, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the formation of contact and intraocularlenses, and more particularly to lens manufacture with computercontrolled machine tools.

For many years, contact lens base curves have been generated by lathinga plastic lens blank while it is grasped in a standard drawback collet.Although this system has proven to be satisfactory in the past and hasbeen used to provide millions of base curves for contact lenses, thereare a number of problems that arise from this operation. Even if thebutton diameter is held to a very tight and repeatable tolerance,interferometer studies show that the base curve is discernibly distortedby the grasp of the collet. The tighter the grasp of the collet, themore distortion. Distortion can be minimized by reducing the grasp ofthe collet. However, going too far in this direction can cause thebutton to slip and as a result, an expensive diamond tool can be ruined.As lathing equipment improves and more accurate base curves are producedby the lathing operation, less polishing is required and generally,better optics are produced (less distortion), if polishing is kept to aminimum.

In order to avoid the distortion produced by having the lens blank in acollet, a number of base curve blocking systems have been developed by avariety of manufacturers of contact lenses in which the blank is adheredto the block with a thick film of wax adhesive. Generally the contour ofthe lens produced from blocked base curves is more accurate and has lessdistortion than base curves generated directly in drawback collets.

However, the use of thick film wax adhesives to block lens blanks beforecutting base curves is not conducive to cutting a series of lenses withconstant center thickness. The wax is usually applied as a thick filmonto the surface of a blocking member and the lens blank applied to thisthick film with pressure which varies from blank to blank. Another typeof prior at base curve blocking member had a cavity on the surface ofthe block that was filed with hot wax. As the wax cooled, it contractedand pulled the lens blank into the cavity deforming the blank anddistorting its shape during cutting of the base curve. When the lensblank with base curve was removed from the surface of the blockingmember, the force is released. The lens blank returns to its originalshape which changes the shape of the base curve.

Several manufacturers produce a constant center thickness base curve(and this is always an additional operation) to eliminate the necessityof measuring center thickness (a difficult operation that is subject toerror) before mounting the base curve blank on a block for front surfacecutting.

Statement of the Invention

The present invention seeks to significantly reduce the possibility oferror in mounting lens blank to a block for generation of the base curvesurface or outer lens surface, to eliminate the need for centerthickness measurement prior to blocking, and to shorten the overallmanufacturing process.

These goals are accomplished by the combination of a new technique forlocating the posterior surface of the lens blank, special blockingmembers that automatically establish the distance to the lens posteriorsurface when the lens blank is mounted, and a modified spindle/colletconstruction that precisely aligns the blocking member (and thus thelens blank) to the spindle.

The base curve blocking member includes a flange with a rearward facingsurface that is spaced a precisely known distance from its forwardalignment surface. The spindle collet can be modified so that it isrecessed into the interior of the spindle, whereby the rearward facingflange surface abuts directly against the forward end of the spindlewhen the blocking member is in place. Since the lens blank is held withits posterior surface at a precisely known location relative to theblocking member, and the blocking member is held at a precisely knowposition relative to the spindle, the position of the posterior lenssurface relative to the spindle is also precisely known without the needto measure the lens blank thickness. Thus, computer controlled formationof the outer lens surface can proceed without the extraneous data entryrequirements of past procedures.

This invention also provides a system for the blocking of lens blankpermitting generation of the base curve profile and a constant centerthickness in a single operation. The button can be mounted on a flatsurface of a base curve lens block parallel to the reference surface bymeans of a strip of adhesive or the block can be provided with a recessaccurately made relative to the rearward facing surface of the flange.In a single setup, an error free base curve can be generated and a knowncenter thickness produced. The subsequent operations are more simple andmore cost effective.

Another aspect of the invention relates to a procedure which uses a verythin film of wax adhesive to provide lenses with constant centerthickness base curves. In accordance with the invention, the flat,polished surface of a base curve lens blocking member is heated to atemperature above the melting temperature of the wax adhesive, usuallyfrom 80° F. to 150° F., generally about 110° F. to 135° F. A film of waxis applied to the surface. A flat surface of the lens blank is placed incontact with the warm film of wax under pressure sufficient to squeezemost of the wax from under the lens blank out onto the surroundingsurface of the blocking member to form a very thin film of wax adhesivehaving virtually zero thickness. The pressure is maintained until thethin film sets and bonds the lens blank to the surface. The pressureapplied to the lens blank is usually a controlled uniform pressure inthe range from 5 to 20 pounds, generally about 10 pounds for 1 to 20minutes.

Surprisingly, the thin film of wax adhesive is found to be stronger thanthicker films. The thinness of the adhesive also assures that the lensblank is mounted perpendicular to the axis of the blocking member andconcentric with the axis of the blocking member with a high degree ofaccuracy. The use of a very thin film of wax adhesive formed underpressure results in reliably cutting a plurality of parts with basecurves with constant center thickness.

The separate measurement of center thickness of each lens on aninterferometer or other instrument is avoided. A batch of lens blankscan be automatically cut with base curves and then processed to finishedlenses without measuring center thickness.

When using a drawback collet to hold the block, the flange of the basecurve block must be probed for position. It is possible to generate abase curve with a known dimension relative to the mounting surface ofthe block. When employing a dead length collet system, a single probingoperation is all that is necessary (providing that the blocks areaccurately made) for a complete run of base curves. Tests indicate thatby very careful blocking, base curve center thickness can be held within±2 microns. Certainly, a ±5 micron center thickness variation for alarge lot of lenses is quite simple to attain by this system.

A base curve with a constant center thickness, mounting ledge, edge liftand the beginning of the lens edge contour can be produced in a singleoperation. As the sag of the lens is known exactly in relationship tothe ledge on the base curve button, it is now possible to mount the basecurve button on a proper block for front surface cutting and to proceedwith the generation of the front surface without measuring centerthickness. Base curve blocking gives the advantages of more accuratebase curves with less distortion, a constant center thickness for costsavings and downstream processing. Further, by utilizing the blockingledge detail the product is prepared for accurate front curve blocking.

Many systems have evolved for the blocking of contact lens base curvelens blanks in preparation for the cutting of the front surface. All ofthe systems are based on using some sort of heated wax-like adhesive andapplying the lens blank to the block with the wax being used as a bond.Systems that utilize wax only, allow for flexure of the lens during thefinal cut and this, of course, results in a lens that is opticallydistorted.

Alignment has been done, in some cases, by spinning the block about itscenter and aligning the button to the block by a probe which controlsthe button (runout). Other systems depend on bringing the lens blank andblock together in some sort of fixture that has been pre-aligned toinsure that the centerline of the lens blank is coincident with thecenterline of the block.

As the lathing process has progressed, pre-cutting of at least a portionof the lens edge, makes it mandatory to improve the runout and wobble ofthe lens blank in relationship to the block.

In the past, hand finishing operations could hide the result of blockingerrors (thick and thin edges) by hand blending processes. As theprocedure for making contact lenses becomes more automated and morecomplete, the requirements for accurate blocking dramatically increase.

A lens blank with base curve and a surrounding ledge self-aligns whenplaced in a front curve mounting block with a complementary surfaceeliminating wobble and providing concentric mounting of the lens blankis disclosed in our prior U.S. Pat. No. 5,205,076.

The block receives the mounting adhesive and the lens blank with basecurve and mounting edge and automatically provides for wobble free,concentric alignment of the lens blank to the block.

The improved front curve blocking system uses a minimal amount of wax.The base curve is supported, below the wax layer, with a portion of theblock made to approximately the same radius as the base curve. Thisprovides proper support during the final cutting of the front curve.

With the front curve blocking system, it is possible to hold the runoutof the button to less than 20 microns TIR. The front curve block mayalso be provided with a flange that aligns and directs the front curveblock and base curve in reference to the spindle. A series of lenseswith the same center thickness can be cut after a single probe to locatethe reference surface on the lens blank.

The base curve blocking member of the present invention may be utilizedwhile forming a reference surface in the lens blank at the same time thebase curve posterior surface is formed, as disclosed in our prior U.S.Pat. No. 5,205,076. The reference surface can be utilized toautomatically align the lens blank with the blocking member and spindlefor creation of the outer lens curve. To accomplish this, the positionof the reference surface is accurately fixed with respect to theposterior surface, and the spindle/blocking mechanism can be modified sothat the position of the blocking member relative to the spindle isaccurately known. This combination results in an automatic alignment ofthe lens blank and spindle prior to the formation of the outer lenscurve, eliminating the need to measure the lens blank after theposterior surface has been formed, and without having to enteradditional data regarding the position and thickness of the lens blankinto the computer control for the lathe system. The automatic alignmentcapability at the same time substantially eliminates discrepancies inboth prism and wobble, setting these factors either at zero or at adesired finite value.

These and many other features and attendant advantages of the inventionwill become apparent as the invention becomes better understood byreference to the following detailed description when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view in section of a first embodiment of a base curve blockin accordance with the invention;

FIG. 2 is a front view in elevation of the base curve block shown inFIG. 1;

FIG. 3 is a side view in elevation of another embodiment of a base curveblock shown assembled with a layer of adhesive and a lens blank;

FIG. 4 is an exploded view in perspective of the assembly shown in FIG.3;

FIG. 5 is a schematic view of the application of heat and pressure tothe assembly of FIG. 3 while received in a spindle of a lathe;

FIG. 6 is a schematic view of the use of a thin film of wax to adhere alens blank to a block;

FIG. 7 is a side view of a lens blank adhered to the block of FIG. 3with a double layered adhesive;

FIG. 8 is a view in section of a cutting system for base curvesproviding constant center thickness after probing;

FIG. 9 is a view in section showing a dead length base curve cuttingsystem for constant center thickness; and

FIG. 10 is a side view of the lens blank with base curve and referencesurface.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in detail in connection with a contactlens having a concave base curve for its posterior surface, although itis equally applicable to intra-ocular lenses having concave, flat orconvex posterior surfaces. The base curve and reference surface can begenerated with a conventional com-puter-controlled lathe system.

A first embodiment of a block 200 for mounting the lens button forforming a base curve is illustrated in FIGS. 1 and 2. The block 200 hasa front flange member 202 connected to a rearward shank 204, having ahollow axial shaft 206. An axial recess 208 is formed in the front faceof the block 200. The recess 208 is sized to receive a lens blank, notshown.

The lower face of the recess 208 contains circular grooves 210 andradial groves 212 forming a cavity for receiving a film of adhesive suchas hot wax adhesive or an ambient temperature curable adhesive. The topedges 214 of the lands 216 remaining after machining the grooves act asa lens block receiving reference surface. The radial grooves 212 mayextend past the outer diameter of the lens button to form runoff cups213 for excess adhesive.

The rear surface 220 of the flange 202 and the front surface 222 of theflange are both orthogonal to the axis of the block 200 and are bothparallel to the reference surface 214. The position of the rear surface220 can be used to locate the reference surface 214 and to program thedepth of cut of the lathe when using a dead end spindle. The frontsurface 222 can be used to probe and locate the position of thereference surface when using a standard drawback collet.

The base curve block 200 can be formed of metal and the front andreference surfaces of the flange can be precisely machined. The use ofthe block eliminates the errors in thickness by directly grasping theplastic button in a collet. However, the diameter and height of thebottom of the lens button can vary. If the diameter is less than therecess the bottom may not be concentrically located and if the diameteris larger than the recess, the button will be distorted. Anotherembodiment of a base curve blocking assembly that is not affected byvariations in the size of the lens button is illustrated in FIGS. 3-6.

The block 300 can be metal or plastic, preferably metal. The block 300has a front flange member 302 connected to a rearward shank 304. Theflange has a front reference surface 322 and a rear surface 320. Theblock 300 does not have a recess. A lens blank 310 is centered byexternal tooling, not shown. The lens blank is attached to the frontreference surface with a thin tape 330 of adhesive, preferably precut tocoincide with the bottom surface of the lens blank. The adhesive tape issolid at room temperature and has higher tensile strength and meltingpoint than the waxes conventionally used for this purpose. The seatingsurface 320 and the flange are in a single plane. The front referencesurface 322 may be probed to allow the lens blank 310 to be cut with aconcave surface and produce a series of parts with a constant centerthickness.

Referring now to FIGS. 3-5, the sticky side 307 of a thin film 330 of athermoplastic tape is applied to the bottom surface 311 of a lens button310. The button-tape assembly is centered on the front surface 322 ofthe flange 302 by a tool 340 having a cup 342 which engages the button310. The assembly is heated by a heat source such as a lamp 313 to atemperature sufficient to soften the film 330 of adhesive while applyingforce from rod 315 to the button 310, suitably 3 pounds of force at atemperature of 130° F. for 30 seconds.

Referring now to FIG. 6, instead of a disc of adhesive, a thin film oflens wax is used. The block 300 is preheated to a temperature of about125° F. from a heat source 350. A film 352 of conventional water solublewax is applied to the central portion of the surface of the blockingmember 300. The lens blank 310 is centered on the front surface 322 ofthe flange and pressure, usually 10 pounds for about 2 minutes isapplied from rod 315 to the cup 342 holding the lens blank 310. Excesswax 354 exudes from under the lens blank 200 onto the surface 322 of theflange 302. The heat source 350 is turned off and the force isterminated after a few minutes. Surprisingly, a thin film of waxadhesive is formed that is stronger than thicker films.

As shown in FIG. 7, the heating step can be eliminated by using a disc400 of double sided adhesive tape. The tape 410 has a substrate 414containing a top layer 402 and a bottom layer 404 of pressure sensitiveadhesive. The disc 400 is adhered first to the lens button 310 and thenthe lens button is applied to the center of the front surface 322 of theflange 302 by tooling, not shown.

Referring now to FIG. 8, a lens button 230 is shown mounted in therecess 208 of the base curve block 200. The shank 204 of the block 200is held by a drawback collet 232 in the nose 234 of a lathe spindle 236.The lens blank is held in the recess 208 by a film 238 of wax adhesive.By probing the position of outer, front surface 222 of the flange 202with a probe 240 to develop a position signal. Controller 242 can beprogrammed to control tool 215 to cut a base curve with a knowndimension relative to the reference surface 217. The standard drawbackcollet 232 requires probing the front surface 222 of the flange 202 ofthe block each time a new block with lens blank is inserted into thecollet 232 in the spindle 234.

FIG. 9 illustrates the base curve block 200 held in a spindle 250 with arecessed collet 252. Since the rear face 220 of the block 200 alwaysabuts the front face 254 of the nose 256 of the spindle 250, thelocation of the reference surface 217 relative to the spindle is alwaysthe same. A single probe of the front surface 222 of the flange 202 isall that is required for a run of base curve turning assuming the blocksare all identical. The cutting of the front surface can proceed withoutmeasuring the center thickness of the lens blanks.

Referring now to FIG. 10, since the base curve 32 and reference surfaces34, 36 are formed in the lens blank 26 during the same manufacturingstage, under a common computer control, and without adjusting theposition of the lens blank 26, the relative positions of these surfacescan be very accurately controlled. The two dimensional factors that areused to automatically align the lens blank during subsequent formationof the outer lens curve are the distance X between the maximum depth ofthe base curve 32 and the top of the surrounding ledge 36.

The invention as described above thus offers a greater degree of controland quality in the manufacture of contact lenses, together with areduction in the number of manufacturing steps and the accompanyingopportunities for error. While specific embodiments have been described,numerous variations and alternate embodiments will occur to thoseskilled in the art. For example, while radial and axial referencesurfaces are preferred for the lens blank, these surfaces could beformed at other angles, so long as corresponding adjustments are made tothe blocking member. Accordingly, it is intended that the invention belimited only in terms of the appended claims.

I claim:
 1. A blocking member to be engaged by the nose extension of aspindle of a lathe containing a recessed collet for carrying a lensblank in which a base curve is to be formed, said lens blank having aflat rear surface, said blocking member comprising:a flange having adiameter larger than the diameter of the collet so as to be able to seaton the forward end of the spindle nose extension, said flange having aforward surface and a rearward surface; a collet engaging shankextending rearward from said flange; a lens blank receiving regionformed in the central portion of said forward surface for receiving saidflat rear surface of the blank; and a reference surface normal to thecentral axis of the flange for locating the rear surface of the lensblank.
 2. A blocking member according to claim 1 in which the flange iscylindrical.
 3. A blocking member according to claim 2 in which theforward surface of the flange is recessed to form a lens blank engagingcavity.
 4. A blocking member according to claim 3 in which the innersurface of the cavity is partially recessed to form wax receiving areas.5. A blocking member according to claim 2 in which the reference surfaceis on the forward surface of the flange.
 6. A blocking member accordingto claim 2 in which the lens blank receiving region is parallel to thereference surface.
 7. A blocking member according to claim 2 in whichthe forward surface of the flange is flat and is perpendicular to theaxis of the flange and forms said flat, lens blank receiving region. 8.A blocking member according to claim 1 formed of metal.
 9. A blockingmember according to claim 8 in which the shank is hollow.
 10. A methodof forming a posterior curve in a lens blank having a flat rearsurface;adhering the flat rear surface of the blank to the center of thefront surface of a flange of a first lens blocking member with a film ofsolid adhesive tape, said flange having a diameter larger than saidblank and larger than the spindle of a lathe and said member having arearwardly extending shank; inserting the shank into the spindle of saidlathe; and turning said spindle relative to a cutting tool to form saidposterior curve in the front surface of the lens blank.
 11. A methodaccording to claim 10 in which the film of adhesive is selected fromthermoplastic adhesive tapes, double sided adhesive tapes and films ofwax.
 12. A method according to claim 11 in which an assembly of the lensblank, thermoplastic adhesive film or wax film and blocking member isplaced under pressure and the assembly is heated for a time sufficientto render the adhesive tacky.
 13. A method according to claim 12 inwhich the film is a wax film and the assembly is placed under a uniformpressure of 5-20 pounds sufficient to expel most of the film of wax fromunder the lens blank to form a thin film of wax and cooling the thinfilm to firmly adhere the lens blank to the front surface of the flange.14. A method according to claim 10 in which a reference ledge is formedin the front surface of the lens blank.
 15. A method according to claim14 further comprising the steps of:adhering the posterior curve andreference ledge to a surface of a second block having a complementarysurface; placing the shank of the second block in the spindle of alathe; and turning the spindle relative to a cutting tool to form afront surface curve on said lens blank.
 16. A method according to claim15 in which the second block contains a central flange having a diameterlarger than the collet of the lathe.
 17. A method according to claim 16in which the rearward surfaces of the first and second lens blockingmembers are placed in engagement with the front nose of the spindleduring said turning steps.
 18. A lens forming system for forming contactlenses or intraocular lenses from a lens blank comprising incombination:a spindle having a bore mounted at its forward end; a firstblocking member having a flange having rearward facing surface a forwardfacing flat surface larger than said lens blank and a shank extendingfrom the rearward surface to be received in said collet; a means foradhering the inner surface of a lens blank to the center of the flatsurface; a means for rotating said spindle relative to a cutting toolfor forming a base curve and a flat annular reference surface at a knownlocation in the lens blank relative to said inner surface of the lensblank; a second blocking member having a first surface complementary tosaid base curve for engaging the base curve of the lens blank and havinga flat annular reference surface complementary to and abutting theannular reference surface and having a rearward directed shank forreceipt in the bore of the spindle having a central flange having adiameter larger than said spindle collet and having a forward directedsurface and a rearward directed surface; and means for rotating thespindle relative to a cutting tool to shape the outer surface of thelens blank to form the outer curved surface of a lens.
 19. A lensforming system according to claim 18 in which the rearward surfaces ofthe flanges of the blocking members abut the front end of the spindle.20. A lens forming system according to claim 18 in which both blockingmembers comprise metal or plastic.